1. Field of the Invention
The present invention relates to an isolated switching power supply apparatus arranged to perform control processing in a plurality of modes including a soft start mode, a sleep mode, a hiccup mode, and a latch mode.
2. Description of the Related Art
In isolated switching power supply apparatuses, in order to use different ground potentials in a primary circuit and a secondary circuit and to satisfy a safety standard, a primary side and a secondary side are isolated using a transformer. In order to control an output voltage and an output current of the secondary side, the output voltage and the output current of the secondary side are detected and are then fed back to the primary side and a switching circuit on the primary side is controlled based on the detected output voltage and the detected output current. At that time, a feedback circuit also needs to achieve the isolation between the primary side and the secondary side.
When a synchronous rectification circuit is used on the secondary side, in order to prevent a case in which a main switch and a commutation-side synchronous rectifier are turned on at the same time and a flow-through current flows, it is necessary to turn off the commutation-side synchronous rectifier in a secondary-side synchronous rectification circuit immediately before the main switch is turned on. Accordingly, a signal indicating a time immediately before the main switch is turned on must be transmitted from a primary side to the secondary side so as to control a time of switching performed by the secondary-side synchronous rectification circuit (see, for example, Japanese Unexamined Patent Application Publication No. 2002-272097). FIG. 1 illustrates a basic configuration of an isolated DC-DC converter disclosed in Japanese Unexamined Patent Application Publication No. 2002-272097.
Referring to FIG. 1, a main switch 2 is subjected to switching control in accordance with a control signal output from a primary-side control circuit 5. The primary-side control circuit 5 detects an output voltage Vo via an isolation circuit 10 and outputs a control signal C1 for controlling a duty ratio of the main switch 2 based on the output voltage Vo. The control signal C1 is also transmitted to a secondary side via driving circuits 7 and 8 and a transformer 9 and becomes a control signal C2. The control signal C2 is supplied to a secondary-side control circuit 21. The control signal C2 is transmitted from the secondary-side control circuit 21 to an input terminal of a driving circuit 13 and a gate electrode of a transistor 15. As a result, a rectification-side synchronous rectifier 3 is driven in phase with the main switch 2, and a commutation-side synchronous rectifier 4 is driven in opposite phase with the main switch 2.
A time difference between the control signals C1 and C2 that is caused by the driving circuits 7 and 8 and the transformer 9 and a delay time of turning off a MOSFET in the commutation-side synchronous rectifier are adjusted by a delay circuit 11 for delaying turning on of the main switch.
As illustrated in FIG. 1, when a synchronous rectification circuit is used on the secondary side, at least two units including a unit for feeding back an output voltage detection signal to the primary side are required to transmit a signal between the primary side and the secondary side while providing the isolation between the primary side and the secondary side. Therefore, a circuit configuration becomes complicated.
As disclosed in Japanese Unexamined Patent Application Publication No. 2002-272097, since a drive voltage Vcc is not supplied to a secondary-side control circuit at the time of activation (soft start) such as power-up, it is desirable that a method of causing a primary-side control circuit to determine a main switch turn-on time and a main switch turn-off time be used. In a sleep mode in which a switching frequency is reduced to reduce a fixed loss under light-load conditions, it is desirable that a method of causing one of the primary-side control circuit and the secondary-side control circuit including an output current detection element to determine the main switch turn-on time be used. Using this method, a switching frequency can be reduced depending on light-load conditions. In particular, when an output current is detected using a drop in voltage at a secondary-side synchronous rectifier or a secondary-side choke coil and a switching frequency is reduced in accordance with the detected output current, it is desirable that a method of causing the secondary-side control circuit to determine the main switch turn-on time be used. In addition, when an output voltage is directly detected and then the output voltage is stabilized by adjusting the duty ratio of a main switch, it is desirable that a method of causing the secondary-side control circuit to determine the main switch turn-off time be used. However, it is difficult to flexibly authorize the primary-side control circuit or the secondary-side control circuit to determine the main switch turn-on time and the main switch turn-off time.